In recent years, digital cameras and camera-equipped mobile telephones have rapidly become common.
A typical digital camera has a function of adding extra information such as a date and a time at which an image is taken, a condition under which the image is taken, and a place at which the image is taken, to the image according to a user's input, and a function of performing image processing on the image according to a user's input designating a compression format, a compression rate or the like.
Here, since digital cameras and camera-equipped mobile telephones are compact, it is difficult to completely prevent photographing an object using digital cameras and camera-equipped mobile telephones at a place such as a museum, where photography is prohibited. Furthermore, “digital shoplifting” at a bookshop has recently emerged as a problem. Here, digital shoplifting means a conduct of photographing, for example, an article in a magazine at a bookshop without purchasing the magazine, using a digital camera or camera-equipped mobile telephone. Japanese patent application publication No. 2001-320702 (a patent document 1) discloses a technique to prevent photographing by means of a digital camera or camera-equipped mobile telephone at a place where photography is prohibited and digital shoplifting, using the above-mentioned functions.
The patent document 1 discloses that a tag which has a blinking infrared light emitting diode (LED) is attached to an object. When a user photographs the object using a digital camera, the digital camera reads a blinking pattern of the infrared LED. Then, the digital camera adds extra information shown by the blinking pattern to an image of the object, or subjects the image to image processing shown by the blinking pattern.
According to this technique, however, the digital camera is required to capture and store a moving image, even when the user desires only a still image, since the digital camera needs to read the blinking pattern of the infrared LED. Therefore, the digital camera needs to be equipped with a memory with a capacity large enough to be able to store a moving image.
Furthermore, the user can intentionally make the blinking pattern unreadable from the digital camera, by covering the tag with something or photographing the object from a particular angle. If such is the case, the digital camera does not read the blinking pattern, and therefore does not perform image processing, even if required.